I realize this could easily be a MASSIVELY stupid question, but my curiosity is stronger than my shame, so here it goes: Is it mere coincidence that the APU outlet of the new Bombardier CSeries looks almost identical to the one on the A380? Are there different APU-outlet-design-philosophies? And if so, what's the reason for some of the out of the usual APU outlet positions and designs, such as on the 777, the CRJ200 or the MD11?

It's all a tradeoff between drag, ease of access, ease of manufacture...

777 for example utilizes screwdriver like tail for aerodynamics efficiency. With 787, Boeing decided that they could make a tip exhaust in such a way, that aerodynamic inefficiency was less of a burden compared with its drawbacks.

The light at the end of tunnel turn out to be a lighted sing saying NO EXIT

The bent exhaust duct on the 777 caused a bunch of problems, the cone is higher drag with simpler design, and the MD-11 has a side exhaust, which IIRC is less safe for rampers. My guess is that the CSeries/380 tail is the best compromise of these considerations.